Question

How long do dead eggs last in the hair?

Answer

Nits(eggs) will not detach on their own because they're wrapped around and held onto the hair shaft with a glue-like substance.  They have to be physically removed unless this glue is dissolved somehow or the hair with the nit attached falls off.  They grow out with the hair, and those nits that are more than a few centimeters away from the scalp are most likely dead or have already hatched.

NitLice eggs are called nits